Pregnant polar bears build snow dens to protect new cubs from the Arctic winter. The researchers found that between 1985 and 1994, 62% of polar bear dens were built on sea ice - but that number dropped to 37% between 1998 and 2004.

"In recent years Arctic pack ice has formed progressively later, melted earlier, and lost much of its older and thicker multi-year component," says Anthony Fischbach of the US Geological Survey (USGS) and one of the research team. "Together, these changes have resulted in pack ice that is a less stable platform on which to give birth and raise new cubs."

Signal drop

In 1985, USGS researchers began putting collars with satellite transmitters onto polar bears to the north of Alaska.

They soon realised the data collected from these collars could provide information on where pregnant females chose to build their dens. The collars transmit less well through the packed snow of a den, causing a noticeable drop in the signal while a female is inside.

Using this method, Fischbach and his colleagues tracked the dens made by 383 females between April 1985 and June 2005. As well as changes in sea ice, they considered other possible explanations for the changing behaviour observed.

However, the team found that female bears only rarely visited the remains of these human hunts. The scientists also ruled out changes in the bears' hunting habits as the reason for the migration.

Thinning ice

The team concludes that more bears are building their dens on land because the Arctic ice is diminishing and becoming less stable. In fact, their satellite tracking revealed that those bears that continued to den on ice tended to shift their dens eastwards, moving away from the western regions where the ice was thinning faster.

The scientists also suggest that thinning sea ice may one day leave the bears stranded at sea, and unable to regain dry land to build their dens.

"Right now, pregnant females foraging offshore in summer must wait up to a month longer than they did just 10 years ago for new sea ice to form so they can travel to denning areas on land," says Steve Amstrup of the USGS. "Alternatively, they must swim ever greater expanses of open water to reach suitable land, or they must den on ice that may not be stable enough to survive the winter."

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Polar bears are having to move their dens to land to raise their cubs (Image: US Fish and Wildlife Service)